PIONEER PLANTS ON A NEW LEVEE.— IV. 



FRANK E. A. THONB. 



The present paper is the fourth of a series of brief notes on 

 succession in the vegetation invading the slopes of a new levee 

 in Des Moinesi, first populated with plants in the spring of 1914.^ 

 The first three papers recorded the events of the first three 

 seasons, the present will note very briefly the conditions during 

 the fourth. 



The record of the first season noted the presence of the pig- 

 weed, Aniaranthus retroflexus, as the dominant plant over the 

 greater part of the area in question, with the exception of cer- 

 tain arid sand-heaps on the opposite side of the river, on which 

 practically nothing grew. During the second season, it was 

 noted that the position of the pigweed was completely usurped 

 by the goosefoot, Chenopodmm albimi, and that the tall rag- 

 weed. Ambrosia trifida, was invading the levee from its origin- 

 ally restricted area at the eastern end of the levee. During the 

 third season the Chenop odium in its turn gave way to the wild 

 lettuce, Lactuea scariola. 



During these seasons also there were changes in the terrain 

 itself. The river, tearing through its new and narrow channel 

 at high water, was eating a wider course for itself, so that by 

 the end of the third season the sand heaps originally noted on 

 the opposite shore had almost wholly disappeared, as had also 

 part of the upper, or southwestern, end of the levee, ne^cessitat- 

 ing certain regrading operations there. In addition the flat 

 ground between the base of the main part of the levee and the 

 lip of the channel was largely engulfed, together with a small 

 part of the main embankment itself. 



The high water preceding the season of 1917 was especially 

 severe, and the erosion -was correspondingly great, so that the 

 river carried away not only the last vestige of the sand heaps 

 mentioned above, and a great part of the soathwestem end of 

 the levee, but a great portion of the main embankment itself. 

 The remaining portion was regraded and in many places filled 

 in with entirely new material, so that as a matter of fact the 

 vegetation of 1917 can not really be said to represent at all a 

 true stage in the succession described in previous papers, but 



'See Proceedings la. Acad. Sri. XXII, 135; XXIII, 423: XXIV, 457. 



